Sputtering of metals in an atmosphere of fluorine and oxygen



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us. cram-191 simi-:enzo or Marais 1N .or enterarme F RUGRKNE ANDGXYGEN iFrank E. Giitery, Aih'son Faris, .Years P. Pressen, Evans City,andrCraries W. Lewis, Pittsburgh', Pa., assigner-s to PPG Industries,Inc., a corporation oi Pennsylvania Fled .lune 29, 1957, Ser. No.649,897

trocondoctive metal oxide films duped with uorine. More specicallyghisinvention pertains to a method of depositing rnetai oxide dln-rs byvsputtering from a metal cathode in the presence of a gaseousdottrine-containing compound. l Y

The deposition of metals and zeta oxide -fdms by cathodic sputtering isa well-known process- 3.5". Patent No. 3,242,006 discloses a processfor'the preparation of tantalum nitride'by cathodic spuf'iering. In sucha cathodic sputtering process, the appiied voltage energizes gaseousions ir. the vacuum chamber and causes such ions to strke'- tbe cathode,thereby displacing a metalV particle. The metal particle mgrates to thesubstrate to be coated, said substrate frequently being an anode. f

In catliodic sputtering processes, a vacuum of greatly reduced pressureis necessary to provide the proper conditions i'or a glowdischarge tooccur between the cathode and the anode. A glow discharge is necessary'to the 1o; claims mail .geteisterd Nov. ii, i3d@ don (i963) at page497. The use of another metal and/'or metal oxide for doping in asputtering process, however,

. involves thev inclusion of such a metal in the cathode.

This, of course, would require cathodes of various compositions where itwasdesired to deposit films ofsimilar thicknesses but of varyingelectroconductivities. This presents process disadvantages. l

Itis also known in the art that a pyrolyzed or ridized metal oxide film,for example, tin oxide, may be ren dared more electroconductive by theinclusion of duorine.

In pyrolytic processes, doping with uorine hasY been accompiished bycontacting a hot substrate with a solution ci a pyroigfzable metalcompound and a pyrolyzable luorine-contaning compoud such as hydrogenfluoride, ammon-ima biluoride, and the like. A process of this type isdisclosedin U.S. Patent'No. 3,107,177. K

Fluonecontaining metal oxide lms have not hetero-r fore beeny producedby cathodc sputtering techniques. Fluorine gas is very corrosive andcannot be eectively handled byvacuum equipment. However, a senttering Yprocess for producing iuorine doped metal oxide films would be moreexible than sputtering processes utilizing a contaminant metal in thecathode as a doping agent.

It has now been discovered that uerine dopedj transparent metal oxidefilms having resistivities as low as 200 ohms/square (specificresistivity of about 0.01 ohm/cms) l or lower may be deposited byinclusion of a uorinecontaining compound in the sputtering cha'mber,said tiuorineconiaining compound being a gas at the operating teneperatnre and pressure of the sputtering process said being capable ofdecomposing in a glow discharge.

energizing of gaseous ions present between the cathode j and the anode.Y One alternate'echniqne for depositing metal oxide films on a substratecomprises contacting a heated substrate with a metal sait orcrganometallic compound which pyr'olyzes at.thc temperature of thesubstrate to form thereon an adherent meta* oxide iim. @ne disadvantageof such v pyrolyzatio'.: procesos resides in the elevated temperaturesnecessary for depositing a suitable metal oxide lm.

When glass is utilized as the substrate, for example, the

necessary pyrolyzation temperamres approach the soften ing point of thegiass, thereby causing undesirable optical distortion in the glass.Cathodic sputtering processes, however, do not require-the substrate tobe heatedto elevated temperatures. Such processes are especiallyamenable,

thereiorefor prodncingtransparent metal oxide lms on `.iiighfxpralityoptical gass substrates.

`metallic films such as tin or copper may have resistivites v of lessthan I ohm/square for similar hn thickness. Thin Ametal oxide lms,however, have much better adhion', i durability, andiight'trazsrnissionthan the pure metal films. One obiective of researchin this are?. hasbeen the vdevelopment of an adherent metaloxidelm which has aconductivity approaching that of thefpuremetal hns.

VOne successful approach towards improvingthe com ductivity of amctaioxide l `.with another metal. generally one cfa rhigher valence. nThe doping ogvactmm deposited tin oxide filmsl withan-:timonytiiasprodnced transparent tin oxidey Elms having has involves*Hoping* 'mgber to musea reaction between the conductivities 1ers than1060 ohms/square. Sudevelop- Dccomposable gaseous uorine compoundsuseulas fluorine Ycarriers in the instant invention include thefollowing lluorocarbons:

monouoromethane diuoromethane triuoromethane fr tetraliuoromethanedifluoroethane triuoroethane Atetratluoroethane i ditiuoroethylene vtetraduoroethylene and the like. The preferred f uorocarbons have fourcar- 'bonatoms voigyless with best results being obtained from thosecompounds having two or less-carbon atoms. Also, it is preferred :thatthe uorocarbons consists of no ele- .ments other than carbon, hydrogen,sulphur, nitrogen,

oxygen, and thehalides. The uorocarbons are derived, generally, byvtluorination of hydrocarbons land are so ,.named, e.g.,tetrauoromethane.

Useful inorganic iiuorine compounds for thepurpose of this inventioninclude:

sulphur hexatiuoride iodine pentauoride nitrpgen triuoride y and thelike'. n

The .preferred iiuorine compounds arethose 'in which.

the decomposition products of the iinorine rcompoundsare gases.`However, nongaseous decomposition products .may be prevented frombecoming occluded inthe deposited iiim 'by reaction wi'thnothielement,forexam- 'l ple, oxygen, to form a gaseous compound. As an illus trationof this tabique, oxygen may be -included. with a uorocarbon in theatmosphere of the sputteringacham- ,cal-bon'r present', rformingcarbonmonoaid or` carbon di' l `ments have by Holland in histex't,Vacuum Y oxide, thereby preventing. the olusonjof gm carbon in.

gen :andany Y amm .Nauru Vposited iilms.

j sputtering apparatus. The sputtering apparatus is coni- ,f posed ot' avacuum cham-ber 2 which contains a cathode vI. which is preferablyconstructed of the metalrwhose oxide is to be deposited on the substrate3. The cathode,

however, maybe constructed of a conductive metal oxide. The substrate V3is supported by a substrate support 4 which may be heated tov improvethe 'j )ropertes of the Y rna tietrirrunnzaiiI affect the lighttransmission ofV the films. The mniting hn would havepropertiesapproachingV that` of a pure mealiic film which has less iight by thetechnique of this inrentiom Especially goed filmsl l. Y ot' oxides of ametal hating an atomic number between 48 and 5l, for example, tin oxide,indiumv oxide, and .Y

metal oxide. The substrate support can be grounded to form an anode.'I'he'cathode 1 is connected to a highvoltage supply 7 and rectifier 9which create `a high'- potential differential between thecathode 1 andthe substrate support 4 (anode). The high-potential differentialprovides.- the glow-discharge necessary to cause deposition of themetallic-oxide from the cathode onto the substrate. TheV vacuum isobtained by a vacuum pump v 5 which exhausts the vacuum chamber to apressure of about miiiitorr or lowg. Higher pressures may be utilized,for example, up Yto 40 millitorr and above;

however, the applied volta-ge may lrequire adjustingfto achieveasuitablegIow-discharge. For the purposes of this invention, thevacuumchamless than about 10-.5 terr, the 'required atmosphere forsputteringis obtained by introduction of a small amount or' the desiredgas which frequently comprises at least a small quantity of an inertgasrln this particular in-r vention, a. reactive gas is introduced -intothe vacuum i v chamber which contains at least one decornposable,Y

gaseous uorine compound. v

A typical procedureV for depositing a metallic oxide application ofabout 2500 volts to the cathode after the system has been evacuated to apressure of about v2i! millitorr. The voltage applied to the system isthat necessary to obtain a suitable glowdischarge and, therefore, willvary with'pressure and the like. A cathode of the dimensions of 12.5centimeters by 12.5 centimeters is positioned millimeters above a glasssample which `is a icentirneter square. The tin cathode is cooled bycooling means 8 .'Jhich is a miniature heat exchanger cooled byintroduction ,oa cooling medium such as a cool gas or a cool liquid. r k

The atmosphere in the vacuum chamber may typical Iy contain about 22percent nitrogen trifluoride, about 26 percent oxygen, and about 52percent argon, although wide variations of atmospherecomposition areuseful. The pressure of the vacuum chamber-.after introduction i, of theappropriate gases should be in the range of about 20 millitorr. Thesubstrate temperature should be maintained in the range of about 360 C.although` temperaturcsas vlow as room temperature may be used.y Operavtransparent conductive ilrnhavng a conductivity of about 1500ohms/square after about 3G minutes of opera- ,ber isequipped with aninlet port 6 for introducing inert yand/or reactive gases into thevacuum chamber. After the proper vacuum is obtained, preferably apressure of Y hn utilizing the above-described .apparatus involves the Ytransmission and less athesion to substrates,l especially l glass, thana metal oxide Various types of oxide films maybe deposited cadmiumoxide, may he deposited by sputtering vin an atmosphere'containing afieconxposable uorine' compound. The conductivity of the resultingiiuorineecontaining metal oxide filmV is man? fold .greater than that'of the undoped' metal oxide film. For example, as indicated above,transparent sputered tin oxidev films' generally have a meanconductivity of about' 2060 ol'lmssqnare.l

However, iinorinefdoped tin oxide ims of similar thick-v nesses havebeen produced by the teaching of this invenl tion with couductivities eiabout 200 ohms/square; a tenfold increase'in the conductiviiyheconductivity being the inverse 'of the resistance of the iilm). Throughoptim- 1 ization o the omi-ating conditions, films of useui'tra'nsntissiouhaving resistivities as low as v1G13. ohms/square or ower areproducinle nveascnable industrial times.

In using the term metal oni e it is intended to niet to the highervalent and lower valent oxides of the metal.

Generally, it is the valent metal oxide which'sv present in thedeposited filnnHowever, substantalquantities of the lower valent oxidemaybe present. For example, tin oxide hns me believed toconsist'primarily of stannic oxide, although minor quantities ofstannous f oxide mayV be present, especially when a deficiency of roxygen exists in the sputtering atmosphere.

, The operating conditions for the @uttering process y of this inventionare similar to those of printer-tsputter-v ing processes. A minimumoltage of about volts isV required to achieve a giow discharge white aminimum voltage of about 100' volts is required to achevea build up ofmetal oxide deposits within a. reasonable time. A

preferred operating voilage above about 1039 voltsfand, for commercialoperations, a'voitage ot' over 1500 volts is recommended.

The operating pressure is generally about 2u millitorr althoughpressures as low as 5 millitorr are useful. Also,

pressures 'of the order o i60 millitorr or higher may besuccessfnllyuuiized. The process is operable atlower utilized, it may bepresent in concentrations of iess than v The invention described khereinis especially-useful in `that it is especially adaptable for producingmetal oxide 1- kearnings of varying conductivities. Films of various,conductivitiesrnay be achieved by varying the concentration of thedecomposable norine compound present in theV f sputtering; atmosphere.Also, it is known vtbatthe oxygen eoncentrationvafiects the conductivityof the resulting` i lnx. Thus, by lowering the oxygencuucentration 'inthe atmospherfthe conductivity'of gt'ne film may be increased. But.'an'increase inwconductivity accompishedbyA a reduction of the amount ofxygenapresent concenuatomtheconcentratiunoftheiinone-containingpressures in the presence of a magne'dc eld. Ihe ultilization of higherpressures involves increased collisions between tli migrating partie* sand the gas atoms of the atmospherex thereby diminishing the rate ofdeposition.

The content of the sputtering atmosphere may be varied considerably Aninert gas, such as argon, nitrogemvor the like, Vmaar or may not, bepresent, if an inert gas. is

1 percent by weightto about 94 percent by weight of the 'total weight ofgases present. 1t is generaliy preferred to have an iner gas prent,preferably in concentrations of about, 5 percent to about 8'! percent byWeight of the gases presmt. The engen concentration may vary from aboutpercent or knature to about 5 grcent or less. As noted above-filmsdeposited in an annosphere of high oxygen concentration tend to havehigh resistivity (low conductivity) while lrns depositedvu an atmospherehaving a low oxygen concentration tend to have greater conductivity buthave less light transmission. For most purposes, `a preferred oxygencomentration is between r about 10 percent and 5 percent by weight ofthe totalA l gases present. The ilumine concentration vmay be also.

varied over wide limits. Tre concentration of norine in the atmospheremay vary from about percent by weight l :toeover 50 percent' by weightoi the atmosphere. A `pn:- V `forged .range ofiuorine concentration isfrom about 3 percent'bl'eigh to met -'vpercent byweight.- Since theconcentralinexpressed n'icrms ofliuornc" utilized.

p `limiting the scope ofthe invention. v i. m

v Exam le I Y uorme-containxng compound. V c

' p 5. The process of claim i whereurthe cathode lstmV variousconciuctivties. The sputtering apparatus included mpond decomposable ina glow discharge is selected 2. The process of clam I. wherein tireatmosphere n, :m a iuorinc compoun v 5 decompcsabie compoonci wiii 4negreater tiran tiiatgiven for the figurine concentration. i

The protuction ofiuonue-:oped mea oxide i'zmsY in a vacuum process isreaiiy' appiicabie to a sputtering comprises an inert gas, oxygedecomposabie in a glow iseharge;

3. The process of ciaim 2 wherein the :nett gas 1s process inasmuch as agovg'ciiscbarge is required. A 5 argon.

. The process of claim 5 wherein the atmosphere consistsessentially ofabout 5 percent to about 87 percent by i f weghty of an inert gas, aboutpercent to about 6G percent by weigbtoxygen, and about 3 percent toabout 40 percent by weight of uorine present as a der:ornposalrxleVterma evaporating Proms could not be com'enientiyA f Thefollowingeztampies iustratie preferred er1-buoi-A ments of thisinvention and shouid not be interpreted asH A sputtering apparatussizm'iar to that illustrated m the met-ar Oxide lmis in oxide FIGURE 1was mhzed m Pmdu tm onde mm i 6. The process of claim 1 wherein thegaseous iiuorine a cooled tin cathode of 12.5 centimeters by 12.5-centimeters. The cathode was positionede millimeters above om the das?consisting of unmcarbons layuig lio more kthan four carbon atoms,sulphur hexatluorxde, rotin-1e peng a dass sam e havin dimensions of 10centimeters b f i gx Y c y intimi-rde, and nitrogen trxuorzde.

10 centimeters square. A voltage of voits D.C. was supplied to thecathode wbiie a gas pressure of about ,2g 7- The Process 0f Cllm 1wherein the sputtering rs mimic was mized in he mmm chamber. Theconducted at a pressure of less than about 100 mrllxtorr.foliowingrtabie illustrates theresuits obtained by deposit- 3 ThePYOCESS Cf dl 1 Where@ the gaseous UUI'ning a metal oxide film in anatmggphge containing ya compound decomposes in a glow discharge to formgasedemposable uorine compound. ons decomposition products. f

TABLE 1.-coMPARrsoN or srufrrnnnn ne AND -FLUOBINE CONTINING TIN OXIDE YGas Composition Flunriue F. Per- Og Per- A Per- R, Time, R',

Sample Compound cent cent cent AzOz ohmlsq. min. ohm-cm.`

A GF4 14.3 14.2 71.5 5:1 550 40 0 012 16.7 63.3 5:1 6,000 60 0 1323---.--- CxHi: 16.7 '33. 0 50.3 1. 5:1 50V 3G O 010 v A Y 4". 60 1.5'19,000 6G 0.198

22.2 7T 8 5:1 350,960 60' 7. 7 25.9 9 2:1 l, 50i] 30 0. 030

r creasing values ofkspecific resistivity indicate increasing thereto,but includes all the vfanau'ons and mooix'ications In the above table,the deposited.. ims were about 9. The sputtering process of claim Ywherein sputter- 2000 A. to about` 2400A. in thickness. The'letter R' 4Ping is achieved lby application of a voltage ofat least .expresses theVspecific resstivityior eachof the lms. Devolts. Y i 1 Y it). Thesputtering process of claim 1 lwherein the s'ub conductivity. v strateis maintained lat a temperature of iess than about Although specificexamples oftlre invention have been 300 C. Y givenv hereinabove, theinvention is not limited soleiy 45 References Cifed falling within thescope of the appended claims. -FOREGN PATENIS.. What is claimed is:830,392 3/1960 Great Britain.

1. A sputtering process for depositing upon a substrate v 1 Y Q aEnorme-containing metal oxicie m of'a metal having 5.9 IOHNH- MACKPnml'y Examiner an atomic number between 4S ane 51 comprising spe er-SLDNEYS, Kugel-ERa Assistant Examine; f ing in vacuo from a metaicathode in an atmosphere com j prising oxygen anda gaseous iimn'necompound `decom- 11S. Cl. X.R. posable in a'glow discharge.

